Network Working Group F. Baker
Internet-Draft
Intended status: Standards Track D. Lamparter
Expires: July 28, 2018 NetDEF
January 24, 2018
IPv6 Source/Destination Routing using IS-IS
draft-ietf-isis-ipv6-dst-src-routing-00
Abstract
This note describes the changes necessary for IS-IS to route IPv6
traffic from a specified prefix to a specified prefix.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Theory of Routing . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Dealing with ambiguity . . . . . . . . . . . . . . . . . 4
2.3. Multi-topology Routing . . . . . . . . . . . . . . . . . 5
2.4. Migration and partial deployments . . . . . . . . . . . . 6
3. Protocol encoding for IPv6 Source Prefix information . . . . 7
3.1. Source Prefix sub-TLV . . . . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Correctness considerations . . . . . . . . . . . . . 10
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
This specification defines how to exchange destination/source routing
[I-D.ietf-rtgwg-dst-src-routing] information in IS-IS for IPv6
[RFC5308][IS-IS] routing environments. To this extent, a new sub-TLV
for an IPv6 [RFC8200] Source Prefix is added, and Multi Topology
Routing [RFC5120] is employed to address compatibility and isolation
concerns.
The router MUST implement the Destination/Source Routing mechanism
described in [I-D.ietf-rtgwg-dst-src-routing]. This implies not
simply routing "to a destination", but routing "to that destination
AND from a specified source". The obvious application is egress
routing, as required for a multihomed entity with a provider-
allocated prefix from each of several upstream networks. Traffic
within the network could be source/destination routed as well, or
could be implicitly or explicitly routed from "any prefix", ::/0.
Other use cases are described in
[I-D.baker-rtgwg-src-dst-routing-use-cases]. If a FIB contains a
route to a given destination from one or more prefixes not including
::/0, and a given packet destined there that has a source address
that is in none of them, the packet in effect has no route, just as
if the destination itself were not in the route table.
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1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Theory of Routing
Both IS-IS and OSPF perform their calculations by building a lattice
of routers and links from the router performing the calculation to
each router, and then use routes (sequences in the lattice) to get to
destinations that those routes advertise connectivity to. Following
the SPF algorithm, calculation starts by selecting a starting point
(typically the router doing the calculation), and successively adding
{link, router} pairs until one has calculated a route to every router
in the network. As each router is added, including the original
router, destinations that it is directly connected to are turned into
routes in the route table: "to get to 2001:db8::/32, route traffic to
{interface, list of next hop routers}". For immediate neighbors to
the originating router, of course, there is no next hop router;
traffic is handled locally.
In this context, the route is qualified by a source prefix; It is
installed into the FIB with the destination prefix, and the FIB
applies the route if and only if the IPv6 source address also matches
the advertised prefix. Of course, there may be multiple LSPs in the
RIB with the same destination and differing source prefixes; these
may also have the same or differing next hop lists. The intended
forwarding action is to forward matching traffic to one of the next
hop routers associated with this destination and source prefix, or to
discard non-matching traffic as "destination unreachable".
TLVs that lack a source prefix sub-TLV match any source address
(i.e., the source prefix TLV defaults to ::/0), by definition.
To ensure that routers without support for Destination/Source routing
are excluded from path calculation for routes with a non-default
source prefix, a separate MTID is used to carry Destination/Source
routes. A router MUST NOT participate in a topology with such an
MTID unless it implements Destination/Source routing.
There is a distinct Destination/Source Routing MTID for each of the
underlying base MT topologies the information applies to. The set of
routes propagated towards the forwarding plane is the union of the
information in the base topology and the D/S Routing MTID. Incoming
connectivity information with a default or non-present source prefix
is advertised in the base topology, routes with non-default source
prefix are advertised in the D/S Routing MTID.
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2.1. Notation
For the purposes of this document, a route from the prefix A to the
prefix B (in other words, whose source prefix is A and whose
destination prefix is B) is expressed as A->B. A packet with the
source address A and the destination address B is similarly described
as A->B.
2.2. Dealing with ambiguity
In any routing protocol, there is the possibility of ambiguity. For
example, one router might advertise a fairly general prefix - a
default route, a discard prefix (which consumes all traffic that is
not directed to an instantiated subnet), or simply an aggregated
prefix while another router advertises a more specific one. In
source/destination routing, potentially ambiguous cases include cases
in which the link state database contains two routes A->B' and A'->B,
in which A' is a more specific prefix within the prefix A and B' is a
more specific prefix within the prefix B. Traditionally, we have
dealt with ambiguous destination routes using a "longest match first"
rule. If the same datagram matches more than one destination prefix
advertised within an area, we follow the route with the longest
matching prefix.
With source/destination routes, as noted in
[I-D.baker-rtgwg-src-dst-routing-use-cases], we follow a similar but
slightly different rule; the FIB lookup MUST yield the route with the
longest matching destination prefix that also matches the source
prefix constraint. In the event of a tie on the destination prefix,
it MUST also match the longest matching source prefix among those
options.
An example of the issue is this. Suppose we have two routes:
1. 2001:db8:1::/48 -> 2001:db8:3:3::/64
2. 2001:db8:2::/48 -> 2001:db8:3::/48
and a packet
2001:db8:2::1 -> 2001:db8:3:3::1
If we require the algorithm to follow the longest destination match
without regard to the source, the destination address matches
2001:db8:3:3::/64 (the first route), and the source address doesn't
match the constraint of the first route; we therefore have no route.
The FIB algorithm, in this example, must therefore match the second
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route, even though it is not the longest destination match, because
it also matches the source address.
2.3. Multi-topology Routing
As outlined in Section 2, this document specifies the use of separate
topologies for Multi Topology Routing [RFC5120] to carry Destination/
Source routing information. These topologies form pairs with a base
topology each as follows:
base base D/S
designated usage MTID MTID
----------------------------------
default topology 0 TBD-MT0
IPv4 management 1 n/a
IPv6 default 2 TBD-MT2
IPv4 multicast 3 n/a
IPv6 multicast 4 n/a
IPv6 management 5 TBD-MT5
Figure 1: Destination/Source Routing MTIDs
The rationale for in-/excluding base MTIDs to provide a D/S MTID for
is as follows:
MTID 0: The base (non-MTR) topology in some installations carries
all routing information, including IPv6 reachabilities. In such a
setup, the topology with MTID TBD-MT0 is used to carry associated
D/S reachabilities.
MTIDs 1 and 3: Topologies with MTID 1 and 3 carry exclusively IPv4
reachabilities. Thus, no IPv6 D/S topology is created to
associate with them.
MTID 2: The topology with MTID 2 carries IPv6 reachabilities in
common M-ISIS setups. (MTID 0 in such cases carries exclusively
IPv4 reachability information.) Associated IPv6 D/S
reachabilities MUST be carried in MTID TBD-MT2.
MTID 4: MTID 4, while carrying IPv6 connectivity information, is
used for multicast RPF lookups. Since Destination/Source routing
is not compatible with multicast RPF lookups, no associated D/S
MTID is defined for IS-IS.
MTID 5: An alternate management/administration topology may carry
its routing information in MTID 5. Destination/Source routing is
applicable to this and MUST use MTID TBD-MT5 to carry associated
reachability TLVs.
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Note that the different topology ID is the sole and only mechanism of
both capability detection and backwards compatibility. D/S routing
will operate correctly if D/S routing information is put in the same
topology as non-D/S information, but adding an IS that does not
support D/S routing will then -undetectably- lead to incorrect
routing decisions, possibly including loops.
Therefore, all routers participating in D/S routing MUST implement
M-ISIS and participate in the appropriate D/S topology per the table
above. Conversely, routers not supporting D/S routing (or not
configured to participate) MUST NOT participate in these topologies.
Even installations that previously used only MTID 0 (i.e. no M-ISIS)
would need to start using M-ISIS on all D/S routers. This results in
correct operation in the face of partial deployment of D/S routing.
Note it is implied by the separate topology that there is a separate
SPF calculation for that topology - using only the participants of
that topology - and D/S routes use paths according to the result from
that calculation. This is an aspect of Multi-topology operation
itself, not this document.
Routers MUST NOT advertise non-D/S routing information using a D/
S-Routing MTID. This includes both reachability information with a
source prefix TLV with value ::/0, as well as without a source prefix
sub-TLV. On receipt, routers MUST ignore any reachability
information in a D/S-Routing MTID that does not have non-default
source prefix information.
To limit complexity, each IPv6 Reachability TLV in a D/S-Routing MTID
MUST have exactly one Source Prefix sub-TLV. Routers MUST NOT
advertise TLVs with more than one Source Prefix sub-TLV, and MUST
ignore any received TLV with more than one Source Prefix sub-TLV.
Systems that use topology IDs different than the values reserved by
IANA should apply the considerations from this section analogously.
2.4. Migration and partial deployments
The Multi-topology mechanism described in the previous section
introduces a distinct, independently operating topology that covers
D/S routers. This easily allows partial and incremental deployments.
Such deployments then contain one or more D/S "subdomains" of
neighboring routers that have D/S routing capability. Since shortest
paths for D/S routes are calculated using a separate topology,
traffic routed on D/S routes will be forwarded inside such a
subdomain until it reaches the router originating the reachability.
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Routers unaware or not participating in D/S routing will in such a
case forward traffic according to only non-D/S routes. This can
produce 2 distinct outcomes:
1. Traffic traverses a D/S router, where a more specific D/S route
matches (and SPF in the D/S topology has found a valid path). It
is then kept inside the D/S subdomain, reaching an originator of
the D/S route.
2. Traffic reaches a system originating a non-D/S route or is
considered unroutable even without regard to D/S routes.
Since the latter case provides no guarantee that there is no D/S
route in the routing domain that could have matched, operators must
pay careful attention to where non-D/S reachabilities are originated
when more specific D/S routes are covered by them.
A very simple configuration that guarantees correct operation is to
ensure covering destination-only reachabilities for D/S routes are
originated by D/S routers themselves, and only by them. This results
in traffic entering the D/S subdomain and D/S routes applying.
Lastly, in partial deployments, disconnected D/S subdomains may
exist. Routers in such a subdomain cannot calculate a path for
reachabilities in a subdomain they're not in. In this case a router
MAY discard packets matching a D/S reachability for which it was
unable to calculate a valid path. Alternatively, it MAY behave as if
the D/S reachability didn't exist to begin with, i.e. routing the
packet using the next less specific route (which could be D/S or non-
D/S). It MUST NOT keep stale SPF calculation results that have
become invalid as result of the topology partition.
This can be remediated by the operator adding connectivity between
the subdomains, for example using some tunneling interface. The new
link is then used to form an IS-IS adjacency fusing the previously
split subdomains. The link will then be used to forward D/S traffic,
possibly incurring some tunnel encapsulation overhead. To the IS-IS
implementation, this link is no different from other links.
3. Protocol encoding for IPv6 Source Prefix information
Destination/Source reachabilities are originated using TLV 237, using
an additional sub-TLV to carry the source prefix as follows.
As noted in Section 2, any IPv6 Reachability TLV that does not
specify a source prefix is functionally identical to specifying ::/0
as the source prefix. Such routes SHOULD NOT be originated into the
D/S MTID, but rather into the base MTID.
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3.1. Source Prefix sub-TLV
The following Sub-TLV is defined for TLV 237:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |Prefix Length | Prefix
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Source Prefix Sub-TLV
Source Prefix Type: TBD-TLV (assigned by IANA)
TLV Length: Length of the sub-TLV in octets
Prefix Length: Length of the prefix in bits
Prefix: (source prefix length+7)/8 octets of prefix
This Sub-TLV MUST occur exactly once in all reachability originated
in any of the D/S topologies listed in Figure 1. A reachability in
these topologies with the Sub-TLV either missing or present more than
once MUST be ignored in its entirety.
4. IANA Considerations
IANA is requested to allocate Values from the "IS-IS Multi-Topology
ID Values" registry as follows:
TBD-MT0: IPv6 Dest/Source routing corresponding to topology 0
TBD-MT2: Reserved for IPv6 Dest/Source routing corresponding to
topology 2
TBD-MT5: Reserved for IPv6 Dest/Source routing corresponding to
topology 5
Additionally, IANA is requested to allocate an IS-IS codepoint from
the "Sub-TLVs for TLVs 135, 235, 236, and 237" registry:
Type: TBD-TLV
Description: IPv6 SADR Source Prefix
Applicable to TLV 237: Yes
Applicable to TLVs 135, 235, 236: No
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5. Security Considerations
The same injection and resource exhaustion attack scenarios as with
all routing protocols apply.
Security considerations from [I-D.ietf-rtgwg-dst-src-routing] are
particularly relevant to this document, in particular the possibility
to inject (more) specific routes to hijack traffic.
6. Privacy Considerations
No privacy considerations apply to this document, as it only
specifies routing control plane information.
7. Acknowledgements
Thanks to Les Ginsberg, Chris Hopps, Acee Lindem, Chris Bowers and
Tony Przygienda for valuable feedback on this document. (TODO:
incomplete, and sort by name.)
8. References
8.1. Normative References
[I-D.ietf-rtgwg-dst-src-routing]
Lamparter, D. and A. Smirnov, "Destination/Source
Routing", draft-ietf-rtgwg-dst-src-routing-06 (work in
progress), October 2017.
[IS-IS] ISO/IEC, "Intermediate System to Intermediate System
Intra-Domain Routing Exchange Protocol for use in
Conjunction with the Protocol for Providing the
Connectionless-mode Network Service (ISO 8473)", ISO/
IEC 10589:2002, Second Edition, 2002.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120,
DOI 10.17487/RFC5120, February 2008,
.
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[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
DOI 10.17487/RFC5308, October 2008,
.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
.
8.2. Informative References
[I-D.baker-rtgwg-src-dst-routing-use-cases]
Baker, F., Xu, M., Yang, S., and J. Wu, "Requirements and
Use Cases for Source/Destination Routing", draft-baker-
rtgwg-src-dst-routing-use-cases-02 (work in progress),
April 2016.
Appendix A. Correctness considerations
While Multi-Topology routing in general can be assumed to work
correctly when used on its own, this may not apply to a scenario
mixing route calculation results as suggested in this document.
However, this specific application is easily understandable as
correct:
Systems that do not implement D/S routing will not participate in
the D/S topology. They will calculate SPF in the base topology.
Packets routed by such system will either (a) cross only non-D/S
routers and reach the last hop as intended, or (b) cross a D/S
router at some point.
For case (b), the D/S router may (b1) or may not (b2) have a more
specific D/S route with a valid path. In case (b2), packets will
be routed based on the same decisions that a non-D/S system would
apply, so they will reach their last hop without any differences.
For case (b1), a break in forwarding behaviour happens for packets
as they hit the first D/S-capable router, possibly after
traversing some non-D/S systems. That router will apply D/S
routing - which, since the path calculation is performed in the D/
S topology, means that the packet is from there on routed on a
path that only contains D/S capable systems. It will thus reach
the D/S last hop as intended.
Packets starting out on a D/S-capable router fall into cases (b1)
or (b2) as if a non-D/S router routed them first.
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For both cases (b1) and (b2), a situation where a D/S router is
aware (by flooding) of a more specific D/S route, but can't
calculate a valid path (because the MT topology is not
contiguous), this is for correctness concerns identical to the D/S
route not existing to begin with. Note below on the correctness
of this.
The compatibility mechanics thus rest on 2 pillars:
D/S routes will match as more specific if applicable
Packets will transit into D/S routing but not out of it
Note that the latter assumption holds true even if D/S routers fall
back to non-D/S paths if they cannot calculate a shortest path
towards the advertising system (either because SPF reaches the
maximum path metric, or because there are multiple discontiguous D/S
subdomains). This is because if a router A receives a packet routed
on a D/S path, this implies the previous router B was able to
successfully calculate SPF, via A, and that A has a path towards the
originating system with a lower path metric than B. Conversely, if
router A is unable to find a valid path, it is safe to assume router
B was unable to do so either, and B forwarded the packet on a path
calculated on non-D/S information.
Lastly, in terms of application use cases, it is also worth pointing
out that loops will always result if (for example on a boundary to an
upstream) the prefix routed incoming to the IS-IS domain is not fully
covered by routes. Just as in non-D/S routing, this may cause a less
specific (default) route to apply and loop packets back onto the same
upstream. With D/S routing, this can now also occur if the incoming
prefix is not covered for all sources. The solution remains the
same: making sure the entire prefix is covered (for all sources),
usually with a discard route. This is not an IS-IS consideration.
Appendix B. Change Log
(to be removed)
Initial Version: February 2013
updated Version: August 2013
Added MTR: August 2014
Split into 4 drafts: October 2014
Dropped 'Critical Sub-TLV' drafts June 2015
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MT clarifications October 2015
Authors' Addresses
Fred Baker
Santa Barbara, California 93117
USA
Email: FredBaker.IETF@gmail.com
David Lamparter
NetDEF
Leipzig 04229
Germany
Email: david@opensourcerouting.org
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